The present invention relates to aqueous dispersions of polyurethanes having (per)fluoropolyether (PFPE) structures, and to their use for the paper sizing.
It is well known that paper is used for the packaging of fats, such as butter or margarine, of compounds such as coffee, chocolate, sugar, and also for the packaging of foods having a high fat content such meats, chips, hamburgers, popcorns, foods to be cooked in microwave oven, crisps for cats or dogs and in general pet food. For said uses it is necessary that the paper is treated with compounds capable to confer resistance to oils, fats and to water (oleo- and hydro-repellence). For these treatments fluorinated compounds (polymers or derivatives thereof having a lower molecular weight) are used.
The oleo-repellence is generally evaluated in terms of resistance of the paper sized to drops of hydrocarbons or mixtures of hydrocarbons having a progressively decreasing surface tension, maintained for 15 seconds in contact with the paper specimen (Kit Test, TAPPI 557 method). Usually as hydro-carbons, castor oil mixtures, toluene and heptane in various ratios are used. The oleo-repellence values obtained by the Kit Test are however an index of the only surface activity of the fluorinated additive, and often they have a poor correlation with the real performance in terms of protective barrier to oils and fats, both in normal utilization conditions of the paper packagings, for example in prolonged contact times with foods, and in severe pressure working conditions (calendering for the obtainment of paper manufactured articles), temperature (heating of the paper manufactured articles in microwave ovens) and mechanical stresses (for example bending to obtain the manufactured article). For this reason the paper users utilize a series of more indicative performanace tests of the practical behaviour in terms of resistance to fats and oils.
The most commonly used tests by the end-users ar the following:                Ralston Crease Test (RP-2 test): The resistance properties to the oil penetration in antigrease papers for pet-food packaging are evaluated. The percentage of the stained paper surface is determined by coloured oil. The acceptability limit of the specimen is 2% of stained surface.        Pet Food Test (crisp test): It is determined the resistance of the paper sized to the direct contact with the crisps used for dog and cat foods, which contain various fats of animal origin, and an amount of water generally not lower than 10% by weight. The combined action of fats, and in this case also of water, together with the mechanical action determined by the combination of the crisp weight and by their shape characterized by pronounced edges, acts so that the test is passed when the paper specimen has a high resistance both to fats and to water. The conditions of said test are more severe than the Ralston Crease Test for the presence of an environment having a high relative humidity and high applied pressure. The test result is expressed as percentage of surface stained with respect to the control. The specimen passes the test if the stained surface results lower than 5% of the surface. The sized paper specimen which passes this test has a high resistance both to greases and to water.        Resistance test to olive oil: The resistance to olive oil is evaluated at 105° C. for 24 hours. The test is passed if the paper is not stained (% of stained surface=0).        Resistance test to butter: The resistance to butter is evaluated in stove at 70° for 30 minutes. The test is passed when, this time elapsed, the paper, visually examined, does not show penetration of melted grease.        
See the Examples for the detailed description of said tests.
Furthermore it is required that the compounds used for the paper sizing give also a certain resistance to water. In fact the pet food test and the butter resistance test require a combined effect of oleo- and hydro-repellence. The test used for the hydro-repellence is the Cobb Test (TAPPI T 441 method). It determines the increase by weight of a paper specimen after the water absorption under standard conditions. The water absorption is expressed as g of absorbed water/m2 of paper. The lower the Cobb test value, the higher the resistance to water of the specimen.
In the field of non polymer fluorinated compounds used for the oleo-repellent paper sizing, phosphates and carboxylates having segments of linear or branched perfluoro-alkyl type, of the CF3—(CF2)n0 — type, wherein n0 is an integer, are known. See for example the publication by R. D. Howells “Sizing with fluorochemicals” presented at “1997 TAPPI Sizing Short Course” 14-16 Apr., 1997. However said non polymer compounds do not give the paper sized high hydro-repellence, especially at high temperature.
Acrylic copolymers containing perfluoroalkyl segments ended with —CF3 groups for the paper treatment are also known. See for example the above publication by R. D. Howells'. The fluorinated oligomeric and polymer structures containing perfluoroalkyl segments with —CF3 type end groups are those considered in the prior art for the use in the paper treatment. See for example the publication “The designing of a new grease repellent fluorochemical for the paper industry”, Surface Coatings International 1998 (9), pp. 440-447. The presence of —CF3 groups is considered in the prior art essential to obtain the necessary resistance to oils and greases. However these compounds even though they show sufficient surface activity to obtain high Kit test values, not always pass the most severe above mentioned performance tests (see the comparative Examples).
Several patents and scientific publications dealing with fluorinated polymers from (per)fluoropolyethers to give oleo- and hydro-repellence to various substrata are known in the prior art, but very few concern the paper treatment. EP 273,449 describes aqueous dispersions of polyurethanes from (per)fluoropolyethers for textile spreadings. The Examples of this patent show that these dispersions give to the substratum a water repellence, but not oleo-repellence. In this patent no mention is made to the use of said polyurethanes for the paper sizing.
EP 533,159 describes ionomeric polyurethanes from (per)fluoropolyethers containing diamines, for applications as consolidating agents for stony or fibrous articles. Tests carried out by the Applicant have shown that these dispersions are unsuitable for giving to the paper sized acceptable oleo-repellence from the industrial point of view.
EP 689,908 describes aqueous dispersions of ionomeric polyurethanes from (per)fluoropolyethers used for the protection of wood, stones and cement from atmospheric and polluting agents. The application as oleo-repellent treatment for the paper is not described. From the Examples the compounds having a higher protective efficacy for wood result to be cationic polyurethanes. Patent application EP 1,059,319 describes cationic polyurethanes from (per)fluoropolyethers having a branched structure and selfcrosslinking. Tests carried out by the Applicant have shown that cationic polyurethanes are unsuitable for the paper sizing, since they give a poor oleo-repellence (they do not pass the above tests).
U.S. Pat. No. 6,224,782 describes polyurethanes containing perfluoroalkyl oligomers and hydrophilic segments based on ethylene oxide units, capable to give to the substratum, in particular textile substratum antistain properties. Said polyurethanes require the presence of significant amounts of surfactants and stirrer which require high shear to be dispersed in water. The high surfactant content can give rise to release and migration phenomena of these compounds when the paper sized with the dispersions of said polyurethanes comes into contact with fat substances. Therefore the polyurethanes of this patent show contraindications when utilized to treat the paper for food use. Besides in U.S. Pat. No. 6,224,782 specific examples of oleo-repellent paper sizing are not described. The use of high shear stirrers to prepare the aqueous dispersions has the drawback to limit the industrial productivity of these polyurethanes.
Furthermore in the industrial application it is preferable that the treatment confers to the paper, in addition to the oil and water resistance, also antiadhesivity properties which favour the release of the semifinished paper sheets in web, allowing to minimize the discarded articles. No fluorinated compound described in the prior art, used as paper additive, mentions said release properties.
The need was felt to have available fluorinated compounds having the following combination of properties:                to pass the most severe performance tests above indicated for oleo-repellence,        good hydro-repellence,        good release effect on the paper sized sheets,        application from aqueous phase to avoid the use of solvents, especially those flammable.        